Earlier quoted context omitted.
I say go simple on the copyright extension formula. If you treat it as a tax (scaling by revenue or similar), it gets messy, encourages dishonesty, and pretty much forces a new subindustry to spring up. But if you keep it simple, you can achieve much the same result without needing more staff in the tax or copyright offices, or inspiring the existence of copyright lawyers or accountants. Ten years of free protection,…
I like your general concept, but my concern with any fixed fee is that it pushes out small publishers in favor of big ones. A small publisher who is making say $300,000 from their work after 40 years can't afford to register it, but Disney can easily afford the 50 year fee. Also, the reason for registration right away is because otherwise you can't be sure a work is in the public domain or who to license it from for…
Registration is a heavy burden that has caused all sorts of pain in the past, and although terms are unreasonable these days I do think that automatic copyright is reasonable. You’re not unsure for ten years, you know for ten years from publication that it’s copyrighted—so you contact whoever wrote it, if you can find them, or you wait and see if not (which would be the case if it was registered anyway, since you can’t predict whether they’re going to renew the thing).
Actually, another argument against requiring registration: it strongly favours big publishers, who know the rules and are used to filling in the paperwork, and makes life harder for individuals, especially in the case of unexpected success (“I didn’t bother registering it because it’s been a waste of time every other time and I didn’t expect it to get popular”). After ten years, you’ll have a fairer idea whether your work is of meaningful commercial value, for which the answer is normally “no”.